The present invention relates generally to circuits, and more specifically to techniques for self-selecting the proper frequency band for use in a multi-band voltage controlled oscillator (VCO) within a phase-locked loop (PLL).
A VCO is a circuit used to generate a periodic signal that may be used for various applications. The VCO signal has a particular waveform (e.g., a sinusoid, square wave, or some other shape) and a particular frequency. Depending on the application being supported, the VCO may need to provide the VCO signal at a predetermined fixed frequency or a range of frequencies.
A common use of a VCO is within a phase-locked loop (PLL). The PLL is able to provide a periodic signal having an accurate frequency based on a reference signal from an accurate signal source, which may be a voltage controlled crystal oscillator (VCXO) or some other oscillator. The PLL locks the frequency and/or phase of the VCO to that of the reference signal by (1) dividing down the VCO signal by a factor of Q, (2) comparing the divided down VCO signal and the (undivided or divided down) reference signal, (3) generating a control signal for the VCO based on the result of the comparison, and (4) adjusting the frequency/phase of the VCO based on the control signal. This mechanism allows the VCO to achieve the frequency accuracy of the reference signal. The PLL is able to provide different frequencies for the VCO signal by using different divider factors for the VCO signal and/or the reference signal.
A VCO needs to be designed with the capability to tune or adjust its frequency over a range that is sufficient to cover a specified range of operating frequencies under expected worst-case conditions. The tuning range denotes the total range of frequencies that may be provided by the VCO with the control signal set at the maximum and minimum values. Depending on the application for which the PLL will be used, the specified operating frequency range may be a single frequency or a range of frequencies. The worst-case conditions may result from different supply voltages, variations in manufacturing process, component tolerances, and so on.
Conventionally, most PLLs employ a single-band VCO that provides a single range of frequencies (or frequency band) for the VCO signal based on the control signal. This single frequency band would then constitute the entire tuning range for the VCO. For a conventional PLL with a single-band VCO, the tuning range of the VCO needs to cover the specified operating frequency range under the expected worst-case conditions. To obtain a large tuning range, the gain of the VCO (which is often denoted as KVCO) may be relatively large. The VCO gain is defined as the change in the VCO frequency versus the change in the control signal amplitude, i.e., the slope of a transfer function for VCO frequency versus control voltage.
A large VCO gain is typically undesirable for several reasons. First, a large VCO gain typically implies that the VCO will be more sensitive to noise, since a small change (e.g., due to noise) in the control signal results in a relatively large change in the VCO frequency. Second, a large VCO gain may result in degraded phase noise on the VCO signal. This is because a large tuning range is typically obtained by using a larger varactor (or variable capacitor). A smaller inductor would need to be used with the larger varactor to achieve a given center frequency since fxe2x88x9d1/{square root over (LC)}. For any given frequency, the quality factor of an inductor is approximately proportional to the inductor value. The signal swing of the VCO is reduced for a lower quality factor, and the smaller signal swing results in poorer phase noise on the VCO signal. The degradation in phase noise may be reduced by designing the VCO with greater drive capability, which would consume more power. However, higher power consumption is undesirable for many applications.
A multi-band VCO provides multiple ranges of frequencies (or multiple frequency bands) that collectively represent the overall tuning range of the VCO. By dividing the tuning range into multiple overlapping frequency bands, the VCO gain may be reduced since the full range of the control signal corresponds to a smaller range of frequencies for one frequency band. The smaller VCO gain of the multi-band VCO may then ameliorate many of the disadvantages described above for the single-band VCO.
Because multiple frequency bands are present in a multi-band VCO, the proper frequency band needs to be selected for use at any given moment. Thus, techniques for self-selecting the proper frequency band of the multi-band VCO are highly desirable.
Techniques are provided herein to select the proper frequency band for use in a PLL from among multiple frequency bands of a multi-band VCO. In an aspect, the band selection is achieved based on a digital representation of the trend of an analog signal to provide improved. This analog signal may be a control signal used to adjust the frequency of the VCO or some other related signal.
A specific embodiment of the invention provides a PLL comprising a detector, a loop filter, a multi-band VCO, and a control unit. The detector receives first and second signals and provides a detector output, with the first signal being related to a reference signal to be locked to and the second signal being related to the VCO signal. The loop filter receives and filters the detector output to provide a control signal. The multi-band VCO receives the control signal and a select signal, selects one of the multiple frequency bands based on the select signal, and provides the VCO signal at a frequency within the selected frequency band determined based on the control signal. The control unit receives a third signal and provides the select signal for the multi-band VCO. The third signal is related to the control signal, and may be a filtered version of the control signal or the control signal itself.
In an embodiment, the control unit includes first and second comparators, a timer unit, and a decision logic unit. The first comparator receives the third signal and a first threshold value and provides a first status signal indicative of whether or not the frequency of the VCO signal is too high. Similarly, the second comparator receives the third signal and a second threshold value and provides a second status signal indicative of whether or not the frequency of the VCO signal is too low. The timer unit provides a timing signal that may include strobes (e.g., pulses) at regular intervals, which may be determined based on the amount of time needed to achieve frequency lock by the PLL. The decision logic derives the select signal based on the first and second status signals from the comparators and the timing signal from the timer unit. The control unit typically evaluates one or more candidate frequency bands to determine the proper frequency band to use, which may be the one that has not changed within a particular amount of time.
Various other aspects, embodiments, and features of the invention are also provided, as described in further detail below.
The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.